Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
A. This procedure includes guidance on duplicating plaster ornament that is chipped or has deteriorated. Plaster of Paris is the most common casting material.
B. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).
A. Rubber Casting Materials:
- Abatron, Inc.
5501 95th Ave.
Kenosha, WI 53144
800/445-1754 or 414/653-2000
- Industrial Plastic Supply
309 Canal Street
New York, NY 10013
- Perma-Flex Mold Company
1919 Livingston Avenue
Columbus, OH 43209
- Polytek Development Corp.
P.O. Box 384
Lebanon, NJ 08833
- Smooth-On, Inc.
1000 Valley Road
Gillette, NJ 07933
B. Rubber Latex: #660:
- Cementex Co.
336 Canal St.
New York, NY 10013
- Gold Bond Building Products Division
National Gypsum Co.
P.O. Box 25884
Charlotte, NC 28229
- U.S. Gypsum Association
810 First Street NE,
Washington, DC 20002
A. Plaster of Paris: "Kal-Kote Finishing Plaster" (Gold Bond, National Gypsum Co.), "Red Top Molding Plaster" (U.S. Gypsum Association), or approved equal.
B. Spackle or patching compound
C. Rubber latex such as #660 (Cementex Co.)
D. Paper towels
F. Clean, potable water
G. Cheese cloth or burlap strips
A. Stiff bristle brushes
B. Heating lamp, electric hair dryer or warm oven
NOTE: VERIFY MIX WITH MANUFACTURER'S INSTRUCTIONS
A. 7 parts plaster
B. 4 parts water
A. Surface Preparation:
- Remove ornamental plaster that is damaged or loose and requires replacement, or that has to be reset.
- Reinforce and secure loose ornamental plaster that is to remain, using appropriate concealed methods.
- Clean plaster surfaces to be restored, to remove loose and deleterious materials that may affect adhesion or application of new plaster. Reattach loose lath or install new lath as required.
- Spray existing plaster to be pointed or patched with water, or use bonding agent, to ensure adhesion of new plaster. Install keying systems for larger pieces as necessary. Comply with referenced standards.
- All existing plaster to be repaired shall be stripped down to a bare finish plaster surface with a non-water based solvent or paint stripper which is not water flushed clean.
NOTE: IT IS NOT ALWAYS NECESSARY TO DETACH THE PLASTER ORNAMENT IN ORDER TO DUPLICATE IT. HOWEVER, IF THE PLASTER ORNAMENT CAN NOT BE REMOVED FROM THE WALL OR CEILING TO MAKE THE MOLD, CURING MAY TAKE SEVERAL DAYS BETWEEN LATEX COATS AND SEVERAL WEEKS FOR THE FINAL CURING.
A. Making the Mold:
- Detach from the ceiling or wall plaster casting to be duplicated.
- Patch the fragment or fill in any damaged portions of the pattern using spackle or other patching compound.
- Carefully clean the fragment for mold making.
- Brush the master piece with the rubber latex.
- Cure the latex at a temperature of about 110oF.; Use an infrared heating lamp, an electric hair dryer, or a warm oven to provide the source of heat; Cure for about 1 hour using heat.
- When the rubber has cured, apply a second latex coating (the number of coats required is dictated by the size of the original and the depth of the relief); Cure as above.
B. Building the Mother:
- The purpose of the mother is to support the mold when it is filled with wet plaster.
- Fill any undercuts in the latex mold with wet paper towels; Lay a thin layer of wet paper towels over the entire surface of the rubber mold.
- Build a frame around the mold to pour the mother into; The mother should be at least 1/2 inch thick at the highest point of the mold.
- Separate the mother from the rubber mold as soon as the plaster has set (in less than an hour).
- Wash the interior of the mold with soap and water and remove any residual byproducts left from the curing process.
C. Mixing the Plaster:
- Add 7 parts by volume of plaster to 4 parts water; Use cold water to retard the set-up time.
- Sprinkle plaster powder in slowly; Stir as little as possible.
D. Casting the Plaster:
- Before pouring plaster into mold, fill the mold with water and then pour water out (this will moisten the walls and ensure penetration of the plaster into all crevices).
- Add plaster and cheesecloth or burlap strips for strength; Jog the plaster-filled mold gently to nudge plaster into all the indentations.
- Level the plaster; Plaster should be hard enough to remove from the mold in 30 minutes; Allow 24 hours to air dry before installing.
E. Install the plaster casting using a thin coat of plaster of paris as an adhesive; Hold the piece in place for a few minutes until the plaster sets; A mastic adhesive or epoxy or gypsum board joint cement are other good adhesives for light pieces.